The present disclosure relates to a semiconductor integrated circuit including a content addressable memory (CAM) and to a technique for reducing power consumption.
In addition to functions for reading and wiring data, a CAM has a search function for searching data stored therein.
To write data to a CAM, the data is transferred to a bit line (BL) and, at the same time, a word line corresponding to the address to which the data is to be written is activated by a row decoder.
In a CAM, searching is made by activating a search line (SL) based on a search data string and determining whether the search data matches entry data in a CAM array. When a matching entry is found by searching, the address of the entry is outputted. An entry stores a data word to be a target of searching and includes plural CAM cells. Each entry is provided with a match line (ML) to which the corresponding CAM cells are coupled in parallel. When a search data string and the data word stored in an entry are compared and are found to match each other, the match line corresponding to the entry is kept at a high level. When, as a result of comparison, it is found that they do not match each other, the corresponding match line is discharged to a low level.
As described, whether the data that matches search data is stored in a CAM can be determined by identifying the potential levels of the match lines.
When, in a CAM, plural entries are found to match search data, the address of one of the entries selected by a priority encoder is outputted.
CAMs like the above-described one are used, for example, for route searching, e.g., as routers and switches, in network devices and for determining a cache miss or hit in a cache memory. For example, a network router includes a CAM used for IP (Internet Protocol) packet routing. The CAM stores IP addresses, and routing is made by comparing an IP address inputted from outside and the IP addresses stored in the CAM. When a match is found as a result of the comparison, data indicating the next destination is written to the IP packet based on the information obtained from the corresponding match line, then the IP packet is sent out.
When search operation is performed in a CAM, all match lines included therein are charged or discharged, so that power consumption of the CAM increases. Also, when the match lines included in a CAM are precharged, it is necessary to reset the search lines to VSS so as to prevent occurrence of a shoot-through current. Namely, every time search operation is performed in a CAM, the search lines are charged/discharged, thereby increasing the power consumption of the CAM. Hence, to reduce the power consumption of the CAM, it is necessary to reduce the power consumption of the search lines and match lines included in the CAM.